In a normally functioning heart, the sino-atrial (S-A) node generates electrical signals that control the contractions of the heart. These signals are generally in the form of quasi-periodic voltage impulses that are of sufficient magnitude to cause the contraction of the heart muscle. In a single cycle of the heart, a signal (i.e., a voltage impulse) is generated by the S-A node, causing the right and left atria to contract. The contractions of the right and left atria force blood into the corresponding right and left ventricles. The signal is also conducted through the AV node to the right and left ventricles (after a short time delay), causing the right and left ventricles to contract.
Various disorders in the cardiac electrophysiological system can cause abnormalities in the rate and/or the timing of the contractions of the heart. For example, a malfunctioning AV node conduction system can delay or prevent the transmission of the signal to the right and left ventricles, impairing or preventing the stimulation of the ventricles. Such disorders can often be corrected by use of a cardiac pacemaker.
Cardiac pacing involves the electrical stimulation of the heart in order to control the timing of the contractions of the heart. Electrical stimuli in the form of pulses are generated by a battery-powered pacemaker and applied to the tissue of the heart by one or more electrodes that are connected to the pacemaker via flexible, insulated conductors. The electrical stimuli supplement or supersede the electrical signals generated by the S-A node. The insulated conductors and associated electrodes form what is referred to as the "lead." The electrode (or electrodes) of a lead may also be used to sense intrinsic electrical activity within the heart.
For endocardial leads that stimulate the inner walls or "endocardium" of the heart, implantation is typically performed by inserting the distal end of the lead into the patient's cephalic vein (one of the main veins from the upper arm), and forwarding the distal end through the subclavian vein and superior vena cava into the right side of the heart. Once the lead is properly positioned (using fluoroscopy to view the distal end of the lead), the proximal end of the lead is connected to the pacemaker and the pacemaker is implanted beneath the skin.
For various disorders it is desirable to pace the heart by applying separate electrical stimuli to the atrium and the ventricle of the right side of the heart. This form of pacing, commonly known as dual chamber pacing, generally requires the placement of an atrial electrode in the right atrium and a ventricular electrode in the right ventricle. In addition to applying electrical stimuli to the tissue of the respective chambers, one or both of these electrodes may be used to sense intrinsic electrical activity, and to thereby detect timing abnormalities. For example, the pacemaker may sense the naturally-occurring electrical activity in the right atrium and use this information to generate appropriate electrical stimuli to apply to the right ventricle.
One problem with dual chamber pacing involves the need to separately position and maintain the atrial and ventricular electrodes in contact with the electrically-sensitive tissue of the respective chambers. The use of separate atrio and ventricular leads permits the independent manipulation and positioning of the atrial and ventricular electrodes. However, the need to pass two separate leads complicates the implantation procedure.
To solve this problem, various designs have been proposed which include both the atrial and ventricular electrodes on a single lead. These types of leads are generally referred to as single-pass atrio-ventricular (A-V) leads.
One type of single-pass A-V lead has a lead body that separates into an atrial branch and a ventricular branch in the vicinity of the heart, with each branch having an electrode at its distal tip. (See, for example, U.S. Pat. No. 4,567,901 to Harris; U.S. Pat. No. 4,643,201 to Stokes; and U.S. Pat. No. 4,393,883 to Smyth et al.) The atrial branch extends into the right atrium, projecting the atrial electrode against the tissue of the right atrium. The ventricular branch similarly extends into the right ventricle, projecting the ventricular electrode against the tissue of the right ventricle (typically in the region of the ventricular apex, as discussed below). One or both branches may be preformed to assume a bent position following implantation. Such bends assist the proper positioning of the electrodes within the atrium and ventricle.
Another type of single-pass A-V lead has both electrodes on a single, unbranched lead body, with the ventricular electrode located at the distal tip of the lead body and the atrial electrode spaced above the distal tip on the lead body. (See Harris U.S. Pat. No. 4,711,027; O'Neill U.S. Pat. No. 4,154,247; and Curry U.S. Pat. No. 4,497,326.) One or more preformed bends in the atrial portion of the lead body may be used to maintain the atrial electrode in contact with the atrial tissue.
With both types of single-pass A-V leads described above, one or more hollow channels or "lumens" may be provided to permit the temporary insertion of one or more relatively stiff guidewires or "stylets" into the lead. When inserted, a stylet maintains the prebent portions of the lead (or the branches of the lead) through which the stylet passes in a substantially straight, but bendable, configuration, facilitating implantation. Upon withdrawal of the stylet (once the distal end of the lead is positioned within the heart), the prebent portions of the lead assume their curved or bent configurations. These bends help to position the electrodes so that they contact the tissue of the respective chambers at desirable locations.
When pacing the heart, it is generally desirable to contact the most electrically sensitive areas of tissue. Contact with such areas permits the use of pulses of lower amplitude and/or pulse width in comparison with the less electrically sensitive areas of the heart. It is also desirable to position the electrodes in areas where they are not likely to dislodge following implantation. The risk of the electrodes dislodging is especially high during the first few hours following implantation, before fibrous tissue of the heart walls forms around the electrodes.
Existing single-pass A-V leads are typically designed to place the ventricular electrode in the right ventricular apex. The ventricular apex has a concave bottom surface that is well suited for holding an electrode in a stable position. Further, the tissue of the right ventricular apex has been found to be suitable for the stimulation and sensing of the right ventricle.
Research with single-electrode leads has shown, however, that stimulation high in the right ventricle, in the region of the right ventricular outflow tract (RVOT), may produce a higher cardiac output. The RVOT is a region high in the right ventricle and below the pulmonary valve. Stimulation within the RVOT, or high along the interventricular septum, is believed to produce a better sequence of activation. The sequence of activation refers to the spread of the electrical signal from it's point of application through the muscle, and the muscular contraction that results from the spread. In comparison to stimulation in the apex, stimulation in the region of the RVOT has been found to produce a more rapid sequence of activation (or QRS sequence), resulting in faster ventricular contractions and improved cardiac output.
Unfortunately, the walls of the RVOT are generally vertical, and are not well suited for the stable placement of a ventricular electrode by conventional methods. Electrode placement in the RVOT is particularly challenging with single-pass A-V leads, wherein both atrial and ventricular electrodes must be maintained in position with a single lead. Accordingly, there is a need for a single-pass A-V lead that is suitable for positioning and maintaining a ventricular electrode in the RVOT.